44 ANATOMY OF AMPHIOXUS. 



function may be, do not serve in any way as balancers ; and, as 

 mentioned in the text, Amphioxus has no means of maintaining 

 its equilibrium when not actually swimming. 



We will therefore keep in mind more especially those Palaeo- 

 zoic fishes which presumably possessed continuous lateral fin-folds 

 serving as balancers. The nearest known fossil relatives of these 

 fishes appear to be the CladoselachidcE (see Bashford Dean. 

 Contributions to tlie Morphology of Cladoselache ( Cladodiis) . 

 Jour. Morph. IX. 1894. pp. 87-112. Also A. Smith Woodward. 

 Tlie Evolution of Fins. Natural Science, I. 1S92. pp. 28-35). 



The lateral fin-folds may be spoken of as mechanical balancers, 

 and to render them efficient organs, there must be a sensory appa- 

 ratus in connexion with them. The suggestion lies near that the 

 ectoderm which took part in the formation of tlie lateral fin-folds 

 also produced the sense-organs of the lateral line. 



The lateral line, through its capacity for receiving impressions 

 of wave-movements, etc., would thus serve as the agent in the 

 co-ordination of such muscular activities as are necessary to the 

 maintenance of the equilibrium. 



Having been once established, no special dififtculty is presented 

 by the fact that the lateral line has spread over the head-region. 

 Moreover, it may be taken as a well-established morphological 

 fact that the auditory organ (internal ear) became evolved as a 

 specialisation of part of the lateral line in the cephalic region, and 

 that it therefore belongs to the same category as the less elaborate 

 sense-organs of the remainder of the lateral line. 



As is well known, the internal ear has two functions, audition 

 and equilibration. It must be supposed that, at its first origin 

 the whole lateral hne served in a general way the function of 

 equilibration, and that this function eventually became chieflv 

 localised in the semicircular canals of the ear, the remainder of 

 the lateral line perhaps undergoing a slight change or limitation 

 of function. 



It seems certain that at first the sense-organs of the lateral line 

 must have been innervated by spinal nerves. This follows both 

 from a priori considerations and also from the condition in Amphi- 

 oxus, where the ectoderm of the metapleural folds is innervated 

 by the Rami cutanei ventrales of the dorsal spinal nerves. Under 



